Still more regarding painting or powder coating springs

Question:

This time you guys have made it incredibly difficult for us readers, what with two outstanding “wrenches” on opposing sides of an issue, namely, chrome plate (and paint/powder coating) on springs.

So with my humble apologies to Richard, I have to raise a voice in support of John Gunnell’s point and even add some emphasis.

My experience with engineering autos and living with the results has left me with vivid memories that it’s never, ever a good idea to chrome plate suspension springs (or apply much of any other kind of plating there). And I understand some reputable chrome shops will refuse to take not only springs but any suspension part as well. Why is it such a HUGE deal? Mr. Prince did highlight the hydrogen embrittlement probability. While there are ways to address hydrogen embrittlement through oven baking the parts after plating and similar post-plating treatments they are certainly not perfect; they are good, not perfect.

What I’m working up to is that if a coil spring snaps it will let you down— instantly—and that’s not good.

If a traditional leaf spring snaps in the main (often only) leaf you and your ride will go down AND around—you’ll lose control of the car without the axle properly controlled.

Same goes for control arms, steering links, track bars and such. That is definitely no good for anybody or any car. So maybe we just plate the non-main spring leafs? Not good either! Any broken leaf may not be real obvious at first but it will put a point load on the leaf above it and create a surface notch as well from the constant digging-in of the sharp, broken end. Springs won’t live with point loads and for sure can’t live with notches on their surface—the fracture routine will eventually propagate to the main leaf, maybe before the owner notices or gets concerned enough to take a thorough look. So that’s the huge deal.

Paint and/or powder coat is also a BIG deal. And ride height has nothing to do with it. Problem is that .040-inch combined thickness of the coating can possibly fret out of the joint area below the leaf spring center bolt. Don’t forget that leaf springs not only flex in a straight up and down (single plane) motion but they also twist due to the differing vertical motion of the axle ends. Right up, left down creates a twist in each leaf spring. (Side note: Hotchkiss axle leaf springs actually act as very effective sway bars in a car due to this twisting motion.) The constant cycling up, down, and twist is what can squish that paint/powder coat (after all, it’s actually just plastic in there) to the point that it frets out from under the clamping surface of the center bolt. And bolts depend on being stretched to do their job (and the same applies to what happens under the U-bolt strap clamping surface).

So that’s my explanation why I would encourage all my fellow car and chrome lovers to find a creative way to make those frustratingly serious parts look pretty. I’m thinking taking the time to mask the mating surfaces on leaf springs prior to painting/coating; maybe some clever covers in the color or shininess of your choice (old-timer cars sometimes used leather wrapping or metal covers both for appearance as well as keeping lube on the leaves).

But please, though the likelihood of hydrogen embrittlement troubles or coating fretting might be low, the damage caused by the potential failure of critical parts is way off the charts.

Thanks to both Richard Prince and John Gunnell on their well-considered advice, great articles and good judgment. I think I can speak for all readers to say we appreciate your help each and every month in AR.

Answer:

Thank you for taking the time to formulate such a well-reasoned explanation and share it with your fellow AR readers. All of your points are well taken but I’m still not convinced that painting or chrome plating spring leaves must be avoided all of the time.

As briefly discussed in the previous response to John Gunnell’s letter (July), chrome plating is remarkably hard and for this reason, among others, it is often used as a durability enhancing surface treatment for various mechanical components, including ones found in critical applications in aircraft, nuclear power plants, and other “high stakes” environments. So I still believe that available techniques to eliminate hydrogen embrittlement and other potentially harmful effects of chrome plating essentially eliminate any added risk of sudden parts failure the plating would otherwise impart.

My biggest concern with chrome plating springs or other critical parts would be whether the necessary processes for negating the negative effects of the chrome plating would actually be carried out correctly. At the risk of sounding like a hopeless cynic, it’s very difficult from a practical standpoint to know whether heat treatment and other needed techniques were done exactly as they should be and that would worry me.

As far as your position that paint on spring leafs can squeeze out, leading to sloppiness where the center bolt holds the leaves together, I understand what you’re saying but don’t think that it presents an insurmountable problem.

First, you cite the 40-thousandths of an inch thickness of paint referenced in the original answer to Mr. Gunnell’s question. This would be an unusually thick coating of paint that is far easier to envision squishing out when compressed between moving steel parts than a much thinner coat.

What you need to consider, however, is that this figure represented the cumulative thickness of the 20 painted surfaces of 10 individual leafs. Intuitively it seems to me that each individual layer of paint that’s no more than 2-mils thick is far less likely to squeeze out.

Beyond intuition, my own experience as a hobbyist and a restoration shop owner has shown no problems with painting spring leafs.

And if not paint, then what? Without some sort of coating, steel will normally rust and when severe enough, rust will impair the integrity of the material. That’s precisely why manufacturers have often painted leaf springs, which by itself reinforces the argument that doing so does not increase the likelihood of a catastrophic failure. And in many cases, whether or not they paint the leafs, manufacturers have often sandwiched some non-ferrous, lubricating material between the steel leafs. Common examples include strips made from plastic, zinc, or even impregnated burlap. Aren’t any of these just as likely as a very thin coat of paint to compress and cause some degree of looseness between the spring leafs?